1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to a polishing apparatus having a workpiece transfer robot for transferring a workpiece from one operation to the next.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical polishing.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring an the turntable are rotated.
The semiconductor wafer which has be polished carries abrasive liquid and ground-off particles attached thereto. Therefore, the polished semiconductor wafer has to be cleaned to remove the foreign matter including abrasive liquid and ground-off particles therefrom by cleaning devices incorporated in the polishing apparatus.
It has been customary to install a workpiece transfer robot to transfer a semiconductor wafer in the polishing apparatus.
FIG. 8 shows a conventional workpiece transfer robot installed in a polishing apparatus. As shown in FIG. 8, a workpiece transfer robot 1 comprises a substantially cylindrical robot body 10, and a pair of arm mechanisms 21 mounted on an upper surface of the robot body 10. Each of the arm mechanisms 21 comprises two articulated arms 25, 27. The arm 25 is operatively connected to the robot body 10 by a joint 35 and the arm 27 is operatively connected to the arm 25 by a joint 37. Hand attachments 33 are operatively connected to the respective arms 27 by joints 38. Hands (not shown in FIG. 8) for holding semiconductor wafers are mounted respectively on the hand attachments 33.
After the semiconductor wafer is polished, abrasive liquid and ground-off particles are attached to the semiconductor wafer. Therefore, the polished semiconductor wafer is transferred to the cleaning device by the workpiece transfer robot 1 and cleaned by the cleaning device. After the polished semiconductor is cleaned, liquid is attached to the semiconductor wafer because cleaning liquid or pure water is applied to the semiconductor wafer. Therefore, when the arm mechanisms 21 are actuated, the cleaning liquid or pure water tends to drop from the semiconductor wafer and the hands, and stays on the robot body 10 and the arms 25, 27.
The liquid tends to be gradually deposited around the joints 35 in greater quantities as shown by hatching parts 39 in FIG. 8, and the liquid enters through gaps in the joints 35 into the interiors of the robot body 10 and the arms 25, thus causing corrosion to internal mechanisms of the workpiece transfer robot 1.
The workpiece transfer robot 1 which has a vacuum chuck includes a vacuum path extending from the interior of the robot body 10 through the interiors of the arms 25, 27 and the interiors of the joints 35, 37, 38 to the vacuum chuck mechanism.
If leakage occurs in the vacuum path in the arm 25, then vacuum will be developed in the arm 25. Even if the joint 35 is sealed, the joint 35 may suffer a seal failure, which results in a leakage for the vacuum developed in the arm 25. Accordingly, the liquids 39 such as water around the joint 35 may be drawn into the interior of the joint 35 under suction caused by the leakage, and may cause corrosive damage to the interiors of the arm 25 and the robot body 10. The same problem occurs at the joints 37, 38.
One solution would be to develop an internal pressure higher than the atmospheric pressure in the arm 25 for driving the water 39 out from the arm 25. However, such a high internal pressure is liable to eject other foreign matter as well as the drawn water 39 out of the arm 25, thus contaminating the exterior of the arm 25, the polished and cleaned semiconductor wafers, and other units such as the cleaning devices.